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“Reprinted from electroindustry magazine by permission of the National Electrical Manufacturers Association.”February 2012

Lately I’ve been seeing a number of specifications for LED luminaires cross my desk. It amazes me how many still reference the old HID “cut-off” designations. Well-intentioned specifications ranging across the spectrum of applications—from roadways to parking lots—still fre-quently call for “full cutoff” despite those designations and associated calculation methods having little to do with LED luminaires.

First, let’s examine cutoff designa-tions.

The term “cutoff” first entered the lighting vocabulary in 1937 as a way to describe a shielding reflector for streetlighting. It has also been used to describe the angle of cutoff in recessed luminaires. For many years, it signi-fied sharp glare control but was not precisely quantified.

In 1972, the Roadway Lighting Com-mittee Recommended Practice (RP-8) defined “cutoff distribution” and “semi-cutoff distribution” with restric-tion on light intensities at vertical an-gles of 80 degrees and 90 degrees above nadir. This was the first time outdoor luminaires were affected by uplight control in a document published by the Illuminating Engineering Society of North America (IESNA)1.

Essentially, the origination of cutoff was devoted to observer comfort or reduc-tion of glare for roadway lighting. Fig-ure 1 shows the effect of the shielding, which is described in veiling luminance (a measure of glare) versus distance. While not a universal truth related to glare, the more cutoff provided, the lower the glare tends to be at farther distances2. The cutoff ratings also did a nice job of describing uplight.

Cutoff Ratings–

LED versus HID LuminairesBy Timothy Hill, Director, Marketing and Product Management—Outdoor, Cooper Lighting

Figure 1. The effect of shielding in cutoff classifications. Courtesy of the Lighting Re-search Center

However, there are two criteria for a fixture to be considered full cutoff:• The luminous intensity (in candelas) at or above an angle of 90 degree above

nadir is zero. • The luminous intensity (in candelas) at or above a vertical angle of 80 degree

above nadir does not numerically exceed 10 percent of the luminous flux (in lumens) of the lamp or lamps in the luminaire.

It’s the second part that causes prob-lems with LED luminaires. In the case of LED, there are no “lamp lu-mens” to consider as the light source is typically an in-tegrated part of the total fixture.

Figure 2 shows a luminaire clas-sification system (LCS) graph (this is the most current method of classify-ing luminaire for uplight and glare) for a typical 400W metal halide lumi-naire that classifies as full cutoff. This

Figure 2. Cutoff for HID fixture

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particular luminaire is also considered a Type IV or forward throw.

This luminaire has no measureable uplight which is great. However, while it doesn’t appear obvious on this chart, in the 80-90 degree critical zone there are 238 lumens. The max candela for this luminaire in the 80–90 degree zone is 2,823. In order to calculate the cutoff rating, 2,823 is divided by 40,000 lumens resulting in 7 percent, which is less than 10 percent, meaning this meets the requirements for full cutoff.

Now consider an LED luminaire of a similar distribution that provides roughly equivalent lighting performance.

Figure 3 LED fixture

This luminaire has no measureable uplight, which is also great. However in the 80–90 degree zone, while it doesn’t appear obvious on this chart, there are 269 lumens, slightly higher than an equivalent HID fixture. The max candela for this luminaire in the 80–90 degree zone is 1,783, or lower than the HID fixture.

Likewise, in order to calculate the cutoff rating, 1,783 would be divided by the total fixture lumens, not the source lumens: 1,783 divided by 17,525 lumens results in 10.1 percent, which is greater than 10 percent, meaning this luminare does not meet requirements for full cutoff.

The conclusion here is obvious—an LED fixture with lower wattage, fewer total lumens, lower glare, and optimal per-formance would not meet a full cutoff specification.

Simply put, cutoff classifications work well for legacy fix-tures. But taking into account the rapid pace of adoption of solid state or LED technology, legacy standards unfairly penalize the newest and greatest.

IESNA has developed a more universal LCS, oftentimes re-

LED versus HID Luminaires . . . Continued from page 28ferred to as “BUG” ratings (B.U.G. = Backlight, Uplight, and Glare), wherein an outdoor luminare can be described more completely and more accurately. In fact, the latest The Light-ing Handbook (available at www.iesna.org) only describes this method. Refer to IESNA TM-15 for a full description.

With 25 years of experience in the electrical and lighting industry, Mr. Hill has held positions in engineering, sales, product management, and marketing.

Lighting Glossary

CFL—compact fluorescent lightsCRI—color rendering indexDALI—digital addressable lighting interfaceHID—high intensity dischargeHPS—high pressure sodiumeHID—electronically ballasted HIDsLED—light emitting diodeLumen—a measure of the power of light perceived by the human eyeSSL—solid state lightingPLC—programmable logic controllers

1 Paulin, Douglas (April 2001). Full Cutoff Lighting: The Benefits, LD&A Magazine

2 McColgan, M., Van Derlofske, J. (2004). Comparison of veiling luminance for roadway lighting. Transportation Research Board 83rd Annual Meeting. Washington, DC, January 11-15

You can’t read the entire sign but it says “Push Button then apply Cal-imine lotine to cross Renner Road”

Submitted by Mike Meeks, Columbus, Ohio who recieved this photo from Steve Sours at the local Eagle Distributor, in Columbus.

Photos of the Month